DEFLUORIDATION TECHNIQUES

BY: TAUFIQUE ANWER

Introduction Fluoride is often described as a „double-edged sword‟

As inadequate ingestion is associated with dental caries.

Where as excessive intake leads to dental, skeletal and soft tissue fluorosis- which has no cure.

The 1984 WHO guidelines suggested that in areas with a warm climate the optimal fluoride concentration in drinking water should remain below 1 mg/l (1ppm or part per million), while in cooler climates it could go up to 1.2 mg/l.

Optimum fluoride concentration in drinking water may be defined as “the one that can arrest the prevalence of dental caries sans causing a insignificant amount of fluorosis”

The concentration of fluoride in water is directly proportional to the degree of fluorosis.

Enamel mottling is widespread in area with water having fluoride content of 3 ppm.

Mottling with discrete pitting of enamel was noticed at fluoride levels of 4 ppm.

Mottling is less in case of fluoride levels of 2.5 ppm to 3 ppm with a dull, chalky, white appearance of teeth.

No motteling or any enamel changes where observed in area with water containing 1 ppm fluoride.

Fluoride level in geographical areas

The total number of people affected is not known, but a conservative estimate by UNICEF would number in the tens of millions

The highest fluoride concentration ever found in natural water was 2800 mg/l, recorded in Lake Nakuru in the Rift valley in Kenya

15 of India's 32 states and union territories were identified by UNICEF as endemic for fluorosis

The main fluoride bearing areas are Gujrat,Rajasthan,Andrha Pradesh where about 50 to 100% of districts are affected.

Methods of defluoridation Providing water, with optimal fluoride concentration

is the only way by which the generation yet to be born can be totally protected against the disease

It can be achieved by the following methods: Removal of fluoride from water (defluoridation),

using suitable techniques. Locating alternative sources of safe water. Bringing in water from a distant, safe source.

Defluoridation Techniques.

Defluoridation was the conventional and widely tested method for supplying safe water to the fluorosis affected communities.

Defluoridation is defined as, „the downward adjustment of level of fluoride in drinking water to the optimal level‟

Defluoridation techniques can be broadly classified in to four categories:

Adsorption technique Ion-exchange technique Precipitation technique Other techniques, which include electro chemical

defluoridation and Reverse Osmosis.

Adsorption technique of defluoridation This technique functions on the adsorption of fluoride ions

onto the surface of an active agent. Activated alumina, activated carbon and bone char were

among the highly tested adsorbing agents. Activated Alumina

Application of domestic defluoridation plant, based on activated alumina, was launched by UNICEF in rural India

The disadvantages with activated alumina are; Adsorption of fluoride is possible only at specific pH range, needing pre-and post- pH adjustment of water.

Frequent activation of Alumina is needed, which make the technique expensive.

Bone char the process of Defluoridation by bone char as the

ion exchange and adsorption between fluoride in the solution.

The efficacy of the plant depends upon temperature and pH of raw water; duration for which the bone-char is in contact with raw water.

It is a highly economic technique with a defluoridation percentage of 62 to 66

Disadvantage :The bone char harbors bacteria and hence

unhygienic. It is a technique sensitive procedure, he use of bone-char may invite cultural and

religious objections

Brick pieces column The basic principle of functioning of Brick piece

column is the same as that of activated alumina. The soil used for brick manufacturing contains

Aluminium oxide.

Mud pot The fluoride removal capacity will vary with respect

to the alumina content The major advantages of mud pots are they are

economic and readily acceptable for the rural communities .

Natural adsorbents Many natural adsorbents from various trees were

tried as defluoridation agents. Seeds of the Drumstick tree, roots of Vetiver

grass and Tamarind seeds were few among them. Researchers at “M. S. Swaminathan Research

Foundation‟ (MSSRF) had shown drumstick seeds to have remarkable defluoridation efficiency, which was higher than that of activated alumina.

Defluoridation by Ion-Exchange technique Synthetic chemicals, namely, anion and cation

exchange resins have been used for fluoride removal. These are commercially produced resins which are

expensive and uneconomical in most circumstancesa) Carbion :it is a cation exchange resin of good durability and

can be used on sodium and hydrogen cycle.

b) defluoron 1:a sulphonated saw dust impregnated with 2% alum solution.

c) Defluoron 2: this was developed in 1968 to over come the problem of defluoron 1. it is a sulphonated coal using aluminium solution at regenerant.

Defluoridation by Precipitation technique

The two major drawbacks of Ion-exchange and adsorption techniques are:

The necessary flow through system is often difficult to arrange where there is no piped water supply

gradual exhaustion of the active agent is not easily detected.

Precipitation methods are based on the addition of chemicals (coagulants and coagulant aids) and the subsequent precipitation of a sparingly soluble fluoride salt as insoluble fluorapatite

The best example for this technique is the famous Nalgonda technique of defluoridation.

Nalgonda technique National Environmental Engineering Research

Institute (NEERI), Nagpur has evolved an economical and simple method for removal of fluoride which is referred to as „Nalgonda Technique‟ (Nawlakhe et al 1974).

The process comprises addition in sequence of sodium aluminate (filter alum), lime and bleaching powder to the fluoride water followed by flocculation, sedimentation and filtration.

This technique is extremely useful for both domestic as well as for community water supply.

Mechanism The unit holds 22 litres of water, which is filled into

the upper chamber. Rapid mix:

Rapid mixing is an operation by which the coagulant is rapidly and uniformly dispersed through out a single or multiple phase system.

It is rapidly mixed for a period of 30 to 60 sec with speed of 10 to 20 rpm so that the coagulant is rapidly and uniformly dispersed.

This help in the formation of micro flocs and result in proper utilization of chemical coagulant.

Flocculation : It is the 2nd stage of the formation of suitable

particles(flocs) from destabilized colloidal size particles.

It is achieved by gentle and prolonged mixing for a period of 10 to 15 min with the speed of 2 to 4 rpm.

Sedimentation : It is the separation from the water by

gravitational setting of suspended particles that are heavier than water.

Factors that affect sedimentation are:a) Size,shape,density and nature of particlesb) Viscosity,density and temperature of water.c) Surface over flow rate.d) Velocity of flow.e) Effective depth of settling zone.

Filtration: It is the process of separating suspended and

colloidal impurities from water by passes through a porous media.

The flocculated water is allowed to settle and filter through fullers earth candle overnight.

Treated water will be available for drinking and cooking with desire level of fluoride.

Maintenance of Nalgonda filters

The package plant installed on hand pump scheme costing Rs 1.6 lakh to serve 250 population.

To serve population of 250, 50 stainless steel filters are required and the cost of 50 filters is approx. Rs 35000.

The main advantage will be its low cost of investment and low cost of maintainence.

Advantages of Nalgonda technique Regeneration of media is not required. No handling of caustic acids and alkalies. The chemicals required are readily available and are

used in conventional municipal water treatment. Adaptable to domestic use. Economical Simplicity of design ,construction,operation and

maintainence.

Can be used to treat water in large quantities for community usage.

Highly efficient removal of fluorides from high levels to desirable levels.

Little wastage of water and least disposal problem. Needs mimimum of mechanical and electrical

equipment. No energy except muscles power for domestic

equipment. Local semi-skilled workers can be readily employed. Simplicity of design, construction, operation and

maintenance.

Simultaneous removal of color, odor, turbidity, bacteria and organic contaminants.

Normally, associated alkalinity ensures fluoride removal efficiency.

Provides de-fluoridated water of uniform acceptable quality.

Disadvantages of Nalgonda technique Desalination may be necessary when the total

dissolved solids exceed 1500 mg/l.

Hardness of the raw water in the range of 200 mg/l to 600 mg/l requires precipitation softening and beyond 600 mg/l becomes a cause for rejection or adoption of desalination.

Generation of higher quantity of sludge compared to electrochemical defluoridation

The large amount of alum needed to remove fluoride.

Careful pH control of treated water is required.

High residual aluminium is reported in treated water by some authors.

Indications of Nalgonda technique Absence of acceptable, alternate low fluoride source

within transportable distance.

Total dissolved solids below 1500 mg/l.

Total hardness is below 600 mg/l.

Raw water fluorides ranging from 1.5 - 20 mgF/l.

Modifications for Nalgonda technique. Poly Aluminium Chloride:

It is evident that for higher concentrations of fluoride, the removal efficiency of fluoride is higher with Poly Aluminium Chloride (PAC) when compared with Alum.

Poly Aluminium Hydroxy Sulphate(PAHS): A polymeric aluminum compound, poly-aluminium-

hydroxy-sulphate(PAHS) is found to require less flocculation time and settling time.

Other techniques of defluoridation Reverse osmosis, electrolysis and electro dialysis

are physical methods that are tested for defluoridation of water.

Though they are effective in removing fluoride salts from water, there are certain disadvantages that limit their usage on a large scale.

Reverse Osmosis and Electro dialysis: In reverse osmosis, the hydraulic pressure is exerted

on one side of the semi permeable membrane which forces the water across the membrane leaving the salts behind.

In electro dialysis, the membranes allow the ions to pass but not the water. The driving force is an electric current which carries the ions through the membranes

Electro dialysis is highly energy intensive and expensive. Both processes are very complicated

Defluoridation by electrolysis: The basic principle of the process is the adsorption

of fluoride with freshly precipitated aluminum hydroxide, which is generated by the anodic dissolution of aluminum or its alloys in an electro chemical cell.

Advantages : Does not require addition of chemicals.

No need to pre & post-treatments .

Low volume of sludge.

Units can be designed for any capacity. Units are designed for specific locations & fluoride

content of water. But can be operated with varying fluoride concentrations by slightly altering the operating parameters.

The electrochemical reactor occupies less floor space. Operator friendly Requires less electric energy (0.3 to 0.6kwh/1000 lts)

Conclusion:

Fluorosis is an important public health problem in India.

Drinking water is the main source of ingestion of

fluoride.

There is no cure to the disease and prevention is the only solution.

Nalgonda technique is the most suitable technique

for Indian rural communities.

Communities should be educated and encouraged to actively participate in the procedure.

Suitable technique for the community should be

identified. Priority should be given to techniques, which utilize

locally available materials as defluoridation agents.

Thank You